Field of the Invention
The present invention relates to fluidized catalytic cracking of hydrocarbon charge stocks. More particularly, it relates to fluidized catalytic cracking wherein a substantial portion of the hydrocarbon charge is converted to hydrogen and wherein said hydrogen is recovered substantially free of contaminant gases, particularly nitrogen.
Fluidized catalytic cracking of hydrocarbon liquids ranging from naphtha through residuum stocks is well known. In such processes, a portion of the hydrocarbon charge is converted to hydrogen and low molecular weight hydrocarbons such as methane and ethane. Although the weight percent of charge converted is small (in the range of 0.5-2 wt. percent), the volume of hydrogen is considerable, considering the low molecular weight of hydrogen and the total weight of hydrocarbon charge to a fluidized catalytic cracking unit. That is, the hydrogen yield from a fluidized catalytic cracking unit charging 100,000 B/D hydrocarbon is in the range of 1.5 to 5 million SCF/D hydrogen. This hydrogen is, however, contaminated with other light gases, such as C.sub.1 -C.sub.2 hydrocarbon, nitrogen, carbon monoxide, carbon dioxide and hydrogen sulfide, and the hydrogen is recovered at a relatively low pressure. Heretofore recovery of the hydrogen as a product has not been economical, but with increased energy costs, such hydrogen recovery is more attractive.
Typically, in fluidized catalytic cracking units, hydrocarbon charge is contacted with hot regenerated catalyst in a reaction zone for cracking the hydrocarbon charge to lighter products. The regenerated catalyst is regenerated with air for burning carbon therefrom and contains nitrogen, carbon monoxide and carbon dioxide entrained therein. As the catalyst circultation rate is high, the amount of entrained nitrogen is substantial. Upon cracking in the reaction zone, hydrocarbon vapors, free of catalyst, flow overhead to a main (or primary) fractionation tower wherein the cracked vapors are fractionated into several liquid fractions and an overhead gaseous fraction.
The overhead fraction comprises hydrogen, nitrogen, carbon oxides, hydrogen sulfide and light hydrocarbons. Commonly, this overhead fraction is compressed to a higher pressure (e.g. in the range of 100-300 psig) and is contacted in an absorber with hydrocarbon liquid wherein a substantial portion of the light hydrocarbons are absorbed such that substantially only C.sub.1 -C.sub.2 hydrocarbons remain in the gas phase. From the absorber, the remaining gas phase flows to an acid gas treater, such as an amine absorber, wherein hydrogen sulfide and carbon dioxide are removed. The remaining gas phase, comprising hydrogen, C.sub.1 -C.sub.2 hydrocarbons, nitrogen and perhaps carbon monoxide is then yielded as a product of the fluidized catalytic cracking process. Commonly this gas product is utilized as fuel gas in the petroleum refinery.
Although it is known to remove light C.sub.1 -C.sub.2 range hydrocarbons from hydrogen streams by such means as cryogenic liquifaction, the nitrogen and carbon monoxide present are much more difficult to remove. Even with the increased value of hydrogen as a product, it is not economically possible to remove the nitrogen and carbon monoxide from the hydrogen.